1. Field of the Invention
The present invention relates to a sound control apparatus of an image forming apparatus which reduces the noise generated by image forming apparatuses such as copying machines, printers, and facsimiles.
2. Description of the Related Art
Conventionally, in image forming and reading apparatuses such as copying machines, printers, facsimiles, and scanners, a cooling fan is used to prevent the increase of temperature of the apparatuses or to discharge the ozone generated in the apparatuses out of them.
In a copying machine having a fixing device and a scanner as a high temperature generating portion in the machine, hot air from the high temperature generating portion is discharged out of the apparatus by the cooling fan provided in an opening portion of a main body. Further, an ozone exhausting fan is provided in the vicinity of a transfer portion and a separation portion in which ozone is generated in the apparatus and ozone is discharged out of the apparatus. Furthermore, a duct is disposed in order to form an exhaust channel from the high temperature generating portion or the ozone generating portion to the outside of the apparatus.
However, the cooling fan and the ozone exhausting fan are placed in the vicinity of the surface of the copying machine body. Thus, noise during operation of the cooling fan and the ozone exhausting fan is directly emitted from the apparatus to the outside, which may cause discomfort to the person near the apparatus body.
The condition in a plurality of the high temperature generating portions is not always uniform. Further, an exhaust passage of an exhaust duct for each high temperature generating portion is not necessarily formed into an uniform shape in view of restrictions of the shape of the copying machine body. Therefore, difference in the cooling efficiency in each of the high temperature generating portions is caused. It is necessary to determine the amount of air in the cooling fan based on the point of measurement which shows the highest temperature in the apparatus. For this reason, some of the high temperature generating portions are overly cooled, which causes the increase of noise in the cooling fan. The same problem is caused in the ozone exhausting fan.
In order to prevent noises of the cooling fan and the ozone exhausting fan, a structure in which sounds (driving sound etc.) in the apparatus are transmitted to the outside of the apparatus via the exhaust duct is proposed. In the structure, a technique that the sounds in the exhaust duct are reduced by detecting the sounds in the exhaust duct and outputting sounds in opposite phase to the sounds into the exhaust duct (active noise control) is used. When the active noise control (hereinafter referred to as ANC) is used, sounds of the cooling fan and the ozone exhausting fan can be reduced in the duct. As a result, sounds emitted from the copying machine to the outside can be reduced (see Japanese Patent Application Laid-Open (JP-A) No. 2002-311763).
The structure in which sounds in the image forming apparatus (driving sound etc.) are transmitted to the outside of the apparatus via the exhaust duct has been exemplary described. Needless to say, when the inside and outside of the image forming apparatus are communicated, the sounds in the apparatus can be transmitted to the outside.
Here, in the ANC, it is necessary that arithmetic processing of the detected sounds is performed by a digital signal processor (hereafter referred to as DSP), the sounds in opposite phase are calculated and output into the exhaust duct by the speaker. Thus, the following problems have been caused.
It is necessary to make a distance between the position which detects sounds in the duct and the position of the speaker which outputs sounds in opposite phase longer. For example, the time for arithmetic processing is 1 ms, the processing time for converting the sounds into a digital sound when taking an analog sound into the DSP is 0.5 ms, and the processing time for converting the sounds into an analog sound in order to output the digital sound of the DSP to the speaker is 0.5 ms. In that case, it is necessary that the total time from when sounds are detected till when they are output by the speaker is at least 2 ms. On the other hand, the sound speed in a temperature of 25° C. is about 346×103 (mm/s) and thus it is necessary that the channel from when sounds are detected till when they are output by the speaker is about 346×103 (mm/s)×0.002 (s)=692 mm.
In the case where the channel length between the sound detecting position and the output position of the speaker is shorter than 692 mm, sounds in opposite phase cannot be outputted from the speaker till when the detected sounds reach the speaker. Thus, it is difficult to reduce the detected sounds. In the structure in which the channel length between the sound detecting position and the output position of the speaker is shorter, it is necessary to use a high-speed and expensive arithmetical element when sounds are reduced.
In order to solve the above-described problems, a method for lengthening a duct channel is considered. However, when the duct channel is lengthened, the size of the copying machine body is disadvantageously increased.
A sound absorbing apparatus having a duct is formed with a first straight path which is continuous with a fan and a second straight path which is continuous with the first straight path via a refracting portion is described in JP-A No. 05-119784. A microphone for detection is provided at the first straight path and a speaker is disposed so as to release a reversal sound out of the second straight path. In JP-A No. 05-119784, the microphone for detection and the speaker are provided in the first straight path. Therefore, in the case of the structure described in JP-A No. 05-119784, when a distance from the microphone for detection to the speaker is ensured to effectively reduce sound by the ANC, the length of the duct from the microphone for detection to the speaker is lengthened and thus the size of an apparatus is increased.